Neglecting any safety issues since this is only going to be used on the race track, I might try a bit different approach.
First weigh the part, then calculate the volume by putting it in a bucket of water. Mark the high water mark and, remove the part and add water to the high water mark using a graduated measuring device (measuring cup). Now you have the weight and volume. Then figure out how much material you would have to remove to get the weight down to the same equivalent volume in aluminum.
In many parts you can remove a lot of material that is not needed in the un-stressed areas by drilling holes and making pockets to form I-beam sections in the stressed areas.
To make this part from scratch from a solid block is not something I would want to do without a CNC, even then it would require a few setups and fixtures. A 4 axis machine would be best.
If you had a 3-axis DRO on your mill, you could use a touch probe to generate a point cloud and plug that into a 3-D cad program to generate the part. Other than that, I'm going the have to agree with
@4gsr that you will need some precision measuring tools.
Unless you are equipped to do high pressure aluminum casting, I would not even attempt to make that part that way. Sand cast aluminum is too porous to make a part with the strength needed. Carving it out of a block of 7075 would be a long process, but possible, even on a manual machine.